not rv but boat?

I believe you are asking about a second vehicle for towing the boat, not double towing the TT & Boat together correct? Towing a 5K boat is way easier than towing a 5K trailer so you won't need near the reserve power that you would towing a TT. You don't have the frontal wind area and the shape of the boat lends itself to better aerodynamics. The biggest thing to consider towing a boat is stopping, surge breaks are no where as smooth as electric and I never felt like they would stop as short as electrics. Second biggie is get 4WD unless you plan on always going to the same ramp and you know it isn't slick. I have been on many ramps that would challenge a 4WD to keep you out of the water.

Have fun, after 28 years with a boat the last 5 without one leaves me lusting for one again!